This invention relates to a device for determining position coordinates of points on a surface. Such a device is often called a digitizer, and this name will hereafter be used for the device.
The prior art of this invention is disclosed in a U.S. Pat. No. 3,647,963 entitled as AUTOMATIC COORDINATE DETERMINING DEVICE. In this prior art, a digitizer comprises a conducting grid structure, a cursor structure having a circular conducting loop element to be moved across the surface of the grid structure, a signal generator which transmits an alternating current excitation signal to the cursor structure, and a signal processor which processes the voltage signal induced in the grid structure. In this digitizer, a coaxial cable must be connected to the cursor to transmit the alternating current excitation signal from the signal generator. In another design of a digitizer, the alternating current excitation signal is applied to the grid structure, and a voltage induced in the cursor coil is connected to the signal processor. In such a design, a coaxial cable must be connected to the cursor to transmit the induced voltage to the signal processor.
Succeeding to the U.S. Pat. No. 3,647,963, there have been several important disclosures including U.S. Pat. No. 3,735,044 entitled as COORDINATE DETERMINING DEVICE EMPLOYING A SLOWLY VARYING DIFFERENCE SIGNAL TO DETERMINE APPROXIMATE CURSOR POSITION. In all these disclosures, the cursor structure has a coaxial cable attached to the cursor.
This coaxial cable attached to the cursor presents an impediment to the maneuverability of the cursor displacement. What is worse, there is a danger of a broken wire accident.
From these reasons, cordless cursor type digitizers which have not a cable connecting a cursor to a digitizer main unit, have been developed and disclosed. In U.S. Pat. No. 4,205,199 a cursor is composed of an input-pen having a magnetic tip to change magnetic permeability of a desired point on a digitizer tablet. For such a cursor, there is not a signal to be transmitted between the cursor and the digitizer main unit. Therefore, this cursor is a cordless cursor.
In the invention disclosed in U.S. Pat. No. 4,451,678, a signal generator and a battery are built-in in a cursor structure. But, this cursor can fulfill its duty when it can induce voltages in grid conductors, and there is no need of an information of a phase of a voltage impressed on the cursor coil (this phase will be called a reference phase) to be used for signal processing. There are several other inventions as disclosed in U.S. Pat. Nos. 4,088,842; 4,497,977; 4,672,154; 4,678,870; 4,697,050 on cordless digitizers in which, similar to the invention disclosed in U.S. Pat. No. 4,451,698, a reference phase information is not necessary in a signal processor.
But, in a type of digitizers as disclosed in U.S. Pat. Nos. 4,185,165; 4,210,775; 46692,568, in which a reference phase signal is used for signal processing, the reference phase signal is transmitted by a cable connected between a cursor structure and a digitizer main unit.
A Japanese Patent Application No. 108685/85 (Provisional Publication No. 267120/86), which is an improvement of the U.S. Pat. No. 4,692,568, discloses a cordless cursor type digitizer. In this cordless cursor type digitizer, conductive lines are scanned in a predetermined time sequence, and as it is known beforehand that a reference phase signal is obtained in an early phase of the scanning, there is no need for transmitting a reference phase signal between a cursor coil and a digitizer main unit.
In a type of digitizers as disclosed in U.S. Pat. No. 3,647,963, in which a cursor coil is placed at an optional point on a grid structure, a reference phase signal can not be obtained by a method similar to the method disclosed by the Japanese Patent Application No. 108685/85.